The science of goniophotometry involves the measurement of the luminous flux emitted by a light source such as a light fixture (termed a "luminaire" in the art). Traditional goniophotometric techniques have modelled luminaires as point sources of light. A rule of thumb commonly used by the lighting industry for nearly a century requires the goniophotometer sensor to be positioned at a distance from the luminaire that is at least five times that of its maximum projected width as seen from the sensor. This is often called the "Five-Times Rule". See: "Relation of Distance to Candlepower Distribution from Fluorescent Luminaires", Illuminating Engineering 47:2, 99-104, C. Horn, W. Little and E. Salter [1952].
Measuring the flux distribution of a luminaire that has been modelled as a point source is referred to as "far-field" photometry. The technique is analogous to the far-field measurements of radio and microwave antennae, albeit with an incoherent radiation source. The measurements are expressed in terms of luminous intensity (candela, or lumens per steradian) as a function of the vertical and horizontal angles of a spherical co-ordinate system centred on the luminaire.
There is no photometric analogue to the "near-field" measurements of radio or microwave antennae, in that diffraction effects play no significant role in luminaire photometry (see for example: "Determination of Far-Field Antenna Patterns From Near-Field Measurements", Proc. IEEE 61:12, 1668-1694, R. C. Johnson, H. A. Ecker and J. S. Hollis [1973]). Nevertheless, it does make sense to refer to the "near-field" flux distribution of a luminaire. An area source with an uneven distribution of luminance across its surface will clearly result in an uneven distribution of illuminance on a plane located in close proximity to the source. This illuminance distribution will vary as the plane is moved away from or towards the source.
The concept of near-field photometry for luminaires has become increasingly important in the lighting industry as attention has focused on the need to evenly illuminate the ceilings of offices equipped with video display terminals. In order to properly design lighting systems for such applications, architects and engineers need to know the three dimensional near-field flux distributions of the luminaires being considered.
Prior art near-field photometric techniques are subject to a number of disadvantages, as hereinafter explained. The present invention is based on a new approach to measuring the three-dimensional near-field flux distribution of a luminaire. It differs from the prior art in that it directly measures the flux distribution, with no reference to the geometry or surface luminance of the luminaire.